Edit by admin: see attached spreadsheet for raw data and full text; some of the spreadsheet text is quoted below...

Quote:

Effect of Tire Size on Honda CR-Z Mileage

Dunlop 195/55R16 SP Sport A/S vs Michelin 205/65R16 Energy Saver A/S

The purpose of the test is to determine whether larger diameter tires, which result in lower engine RPM and higher load for a given speed, can improve gas mileage. The test car has a 6-speed manual transmission. The car is also available with a CVT transmission. The CVT equipped car has a higher EPA mileage rating, and is known to run at lower RPMs at highway speed. All tests were run using Honda OEM 16” wheels.

Conclusions: At highway speeds (55-65 MPH), the larger tires gave a 5.7% average increase in MPG. The engine ran about 200 RPM lower, giving a quieter ride, and theoretically, less engine wear. This is balanced by requiring a little more clutch slip on starting, resulting in more clutch wear.

On a slightly different note: 10 more MPH costs 10 MPG!

The test course consists of northbound and southbound runs of 12 miles each on US Route 23, in Marion County, Ohio. The start is at mile marker 3, and the end is mile marker 15. Each data set consists of one north+south run at 55 mph, one at 60, and one at 65 mph. The distance is assumed to be correct, as I'm using state highway markers. Each run was timed with a stopwatch, and the MPG was read from a ScanGuage II. The ignition was left on between north and south runs, so that the cruise control should be exactly the same setting each way. All tests were run with the tires at 40 PSI cold. Warm up consists of 7 city blocks at 25 MPH (4 stop signs), 1.5 mi. at 45 MPH (3 stop lights), and 7.1 mi. at 55 MPH (4 stop lights)